This invention is related to an improved apparatus for supporting and rotating a down hole tubular in an earth boring apparatus.
One type of known earth boring apparatus includes a derrick which supports an apparatus which can be used to support and rotate down hole tubulars such as drill pipe. This apparatus, which is commonly known as either a top head drive or a swivel, is movable up and down along the length of the derrick. In addition, the swivel commonly includes a drilling fluid seal which conducts drilling fluid from a non-rotating conduit to a rotatable shaft included in the swivel, which shaft, in turn, can be connected to a string of drill pipe to pass drilling fluid from the non-rotating conduit to the drill pipe.
In that the swivel must support the entire weight of the drill string, it must be constructed to pass heavy downward loads to the derrick without excessive deformation. In addition, the swivel must apply large torques to rotate the drill string and conduct these torques to the derrick. The combination of large downward and large rotary stresses presents several problems which can result in damage to the swivel and the derrick if not properly overcome.
First, down hole tubulars such as drill pipe are often not perfectly straight. When rotated by the swivel, such crooked tubulars can exert large skew forces on the swivel as the axis of rotation of the tubular shifts slightly in each cycle of rotation. In the past, these skew forces have resulted in chipped or spalled bearings in the swivel.
Second, rotary drive units and drilling fluid seals generally require precise alignment with the swivel shaft if they are to operate reliably. Eccentricities between the swivel shaft and the drive unit can result in undesirable wear and breakage in the drive unit, and eccentricities between the swivel shaft and the drilling fluid seal can result in undesirable leakage of drilling fluid as it leaves the non-rotating conduit and enters the swivel shaft.